The acceptance of one’s fast is dependent on God

Since Thou hast adorned them, O my Lord, with the ornament of the fast prescribed by Thee, do Thou adorn them also with the ornament of Thine acceptance, through Thy grace and bountiful favor. For the doings of men are all dependent upon Thy good-pleasure, and are conditioned by Thy behest. Shouldst Thou regard him who hath broken the fast as one who hath observed it, such a man would be reckoned among them who from eternity had been keeping the fast. And shouldst Thou decree that he who hath observed the fast hath broken it, that person would be numbered with such as have caused the Robe of Thy Revelation to be stained with dust, and been far removed from the crystal waters of this living Fountain. (Baha'u'llah, 'Prayers and Meditations by Baha'u'llah')